Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
U.S. Pat. Nos. 5,914,709, 6,181,842 and 6,351,260, and U.S. Patent Application Nos. 2002/0088930 A1 and 2004/0201579 A1 (the contents of which are incorporated into this specification by way of cross-reference), describe an optical touch screen sensor in which planar optical waveguides are used to launch an array of light beams across a screen, then collect them at the other side of the screen and conduct them to a position-sensitive detector. The touch screens are usually two dimensional, with two arrays (X, Y) of send waveguides along adjacent sides of the screen, and two corresponding arrays of receive waveguides along the other two sides of the screen. As part of the “transmit side”, in one embodiment a single optical source launches light into a plurality of waveguides that form both the X and Y send arrays. In another embodiment, the X and Y send arrays each receive light from a separate optical source. U.S. Patent Application No. 2004/0201579 teaches that the light beams launched across the screen preferably form a “lamina” (i.e. a thin sheet) of light of substantially uniform intensity. This minimises the required dynamic range of the photodetectors associated with the X, Y receive arrays, and enhances the grey scale interpolation of the position sensing algorithms. Accordingly, with either the single or dual optical source embodiment, it is preferable to have equal 1×N splitting of power from the optical source into the corresponding array of N waveguides. It will be appreciated that the spatial resolution of this type of optical touch screen is determined by the number of beams traversing the screen, which corresponds to the number of waveguides in the transmit and receive arrays. However since the waveguide arrays need to fit within the bezel of the screen, there is only room for a limited number of waveguides within each array, representing a limitation of waveguide-based optical touch screens.
Optical waveguides that confine light in two dimensions (such as optical fibres or channel waveguides) may be either singlemode or multimode. In singlemode waveguides, optical power propagates in a single, well defined mode with an intensity profile that is approximately Gaussian in shape. Although this is an approximation, it is mathematically convenient and frequently used when modelling optical waveguides. Hereinafter, the intensity profile of light in a singlemode optical waveguide will be referred to as Gaussian in shape. As shown in FIG. 1, if the confinement in one dimension is lifted when a singlemode channel waveguide 10 joins a slab region 11, the Gaussian mode 12 diffracts in the unconstrained dimension to produce an expanding wavefront 13 that maintains a Gaussian intensity profile. In multimode waveguides on the other hand, optical power propagates in at least two, more often many, modes, with an overall intensity profile that is more complicated than a simple Gaussian. Upon entering a slab region, the optical power diffracts into a wavefront that is non-Gaussian in shape.
Integrated optical devices capable of equal 1×N splitting of optical power are known for the case of singlemode waveguides. One class of such devices known in the art are multimode interference splitters, commonly known as MMI splitters (L. B. Soldano and E. C. M. Pennings, IEEE Journal of Lightwave Technology vol. 13 No. 4, pp. 615-627, April 1995). As shown in FIG. 2 for the specific 1×4 case, a 1×N MMI splitter 20 comprises a singlemode input waveguide 21, a multimode section 22 and N singlemode output waveguides 23. N is generally a multiple of two, and for equal splitting the input waveguide should be centrally located on the input face of the multimode region and the output waveguides evenly spaced along the output face. By correctly choosing the width and length of the multimode section 22, the light emanating from the input waveguide 21 is “self-imaged” to form N images at the entrances to the output waveguides. Although in theory N can be any multiple of two, practical considerations such as device length and fabrication tolerances limit MMI splitters to low port counts such as 1×2, 1×4 or 1×8. Furthermore, because MMI devices rely on interference effects to produce well defined self-images, they are only applicable to singlemode inputs.
Equal 1×N splitting with much higher port counts, or for port counts that are not a multiple of two, can be achieved with a “tree” splitter comprising an input waveguide, a diffractive slab region and N output waveguides, where the output waveguides are generally located on a circular arc centred on the input waveguide. A 1×8 tree splitter 30 is illustrated schematically in FIG. 3, where a singlemode input waveguide 31 launches a Gaussian beam 32 into a slab region 33. This beam diffracts as it propagates through the slab region, producing an expanding Gaussian wavefront 34 that impinges on and is coupled into an array 35 of output waveguides. Since the amount of light coupled into each output waveguide depends approximately on the optical field integrated over the cross sectional area of the waveguide, equal power splitting is achieved when the width of the waveguides increases progressively away from the centre (where the Gaussian profile is peaked). 1×N splitters of this type are disclosed by S. Day et al. “Silicon based fibre pigtailed 1×16 power splitter”, Electronics Letters vol. 28 No. 10, pp. 920-922, 7th May 1992, and in Japanese patent application No. JP6138335A2. Note that for ease of fabrication, the output waveguides (and the slab region) are constrained to have equal height, so to adjust the cross sectional area for the overlap integral, the width needs to be varied. Note also that to maximise the amount of light coupled into the output waveguides, the gaps 36 between the output waveguides should be made as small as possible. However because any given fabrication technique has a resolution limit (i.e. the smallest structures that can be patterned), these gaps generally cannot be made arbitrarily small.
1×N tree splitters are a special case of M×N star couplers, also well known in the art (see for example U.S. Pat. No. 4,904,042), where M input waveguides and N output waveguides are located on opposite sides of a diffractive slab region. The input and output waveguides are generally singlemode, although tree splitters and star couplers with multimode waveguides are also known in the art (see for example U.S. Pat. Nos. 4,484,794 and 6,021,243). However in known multimode waveguide star couplers, the input and/or output waveguides are invariably uniform in width, with no attempt made to tailor the distribution of widths to match a certain intensity profile in the slab region. For highly multimode waveguides supporting hundreds or possibly thousands of modes, this approach is acceptable because the wavefront diffracting into the slab region from a highly multimode input waveguide will be more or less uniform in intensity, resulting in more or less equal splitting into an array of identical output waveguides.
There is another regime, which can be termed few-moded waveguides, situated between single mode and highly multimode waveguides that support in the order of two to a few tens of modes. In the case of a few-moded input waveguide, the diffracting wavefront in the slab region will have a non-Gaussian intensity profile that may nevertheless be well-defined, and to achieve equal splitting, the output waveguide widths should preferably follow a corresponding “inverse” or “complementary” profile. For waveguide-based optical touch screens of the prior art, various aspects of system design mean that multimode or few-moded waveguides are preferred. However to the best of our knowledge, no devices for achieving equal 1×N splitting of optical power from a few-moded input waveguide are known in the art. Accordingly, it would be desirable to provide an integrated optic 1×N splitter that achieves substantially equal splitting of a non-Gaussian beam launched from an input waveguide into a slab region by appropriately tailoring the widths of the output waveguides. It would be further desirable to provide, in a waveguide-based optical touch screen sensor, an integrated optic 1×N splitter that achieves substantially equal 1×N power splitting of light from a single input waveguide.
The above discussion has dealt with devices for 1×N splitting where light enters the slab region from an input waveguide. A different situation arises if the input waveguide is dispensed with, and light launched directly into the slab from an optical source. To achieve equal 1×N splitting of optical power, the distribution of output waveguide widths may still need to be tailored such that, for a particular intensity distribution of light diffracting in the slab region, the overlap integral of the optical field and waveguide cross-sectional area is equal for each output waveguide. Accordingly, it would be desirable to provide an integrated optic 1×N splitter that achieves substantially equal splitting of a non-Gaussian beam launched from an optical source into a slab region by appropriately tailoring the widths of the output waveguides. It would be further desirable to provide, in a waveguide-based optical touch screen sensor, an integrated optic 1×N splitter that achieves substantially equal 1×N power splitting of light launched directly from an optical source into a slab region.
A particularly favourable situation arises if light can be launched into the slab region such that it excites a substantially uniform or “top hat” intensity distribution, as the output waveguides can then have equal widths. It would be therefore desirable to provide a means of achieving substantially equal 1×N splitting of optical power by directly exciting a substantially uniform intensity distribution in a slab region with an appropriate optical source.
If equal intensity splitting is required with a 1×N tree splitter, then it is impossible for all the power to be captured, irrespective of the particular intensity distribution (i.e. Gaussian or otherwise). This is because every physical intensity distribution tails off to infinity, and if each output waveguide were to receive 1/N of the input power, then the outermost output waveguides would need to have infinite width. This is generally not practical, so the outer edges of the intensity distribution have to be neglected or discarded. This is ensured by designing the slab region to be wider than the array of output waveguides connected thereto (i.e. where they connect to the slab region), so that the outer edges of the intensity distribution fall outside the waveguide array. Equivalently, the (generally curved) end face of the slab region is longer than the sum of the output waveguide widths and intervening gaps. This is known in the art for the singlemode case, as noted in JP6138335A2 (tree splitters) and U.S. Pat. No. 4,904,042 (star couplers). However to the best of our knowledge, such devices with multimode waveguides are routinely designed such that the input and/or output waveguides completely fill the slab apertures (see for example U.S. Pat. Nos. 4,484,794 and 6,021,243). Again this will be generally acceptable for highly multimode waveguides where the wavefront diffracting into the slab region will be more or less uniform in intensity with minimal power in the tails. In the case of a few-moded input waveguide however, the diffracting wavefront in the slab region may have a non-Gaussian but still well defined intensity profile, with significant power in the tails that will have to be discarded for equal splitting. Similarly to the singlemode case, this can be done by designing the slab region to be wider than the array of output waveguides connected thereto. A second benefit of such a design is that it prevents any reflections of the diffracting wavefront off the slab region side walls. These reflections can interfere with the main diffracted beam causing interferometric peaks and troughs in intensity, resulting in speckle in the power distribution at the end of the slab region on a length scale comparable to the waveguide dimensions. In the 1×N tree splitters of the present invention, it is therefore preferable that the diffractive slab region be wider than the array of output waveguides connected thereto.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.